CROSS REFERENCE TO RELATED APPLICATION
[0001] The present application is related to applicant's co-pending application filed simultaneously
herewith as Serial No. on for "Method For Temporarily Sealing Holes In
Printed Circuit Boards" and is incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to the manufacture of printed circuit boards and more
particularly to an apparatus and method for temporarily sealing holes in the printed
circuit board laminate during processing.
[0003] In the manufacture of printed circuit boards, a photoresist is used to transfer the
outline of the circuit into the copper surface of the board. The name photoresist
defines the dual functioning nature of this material. First it is a photo polymer
whose chemical properties are changed by exposure to ultraviolet radiation. That exposure
is done selectively through a mask outlining the circuit being defined. The dual functioning
comes into play after developing the photo-polymer, where the soft unwanted areas
are washed off the copper surface. What remains is a protective covering of hardened
polymer only in those areas outlined by the exposure mask. In one application this
protective covering resists the etching process so that only the copper left unprotected
is etched away. When the resist is finally removed, the protected copper circuit lines
underneath become the electrical conductors of the circuit board.
[0004] One real measure of the evolution of printed circuit board technology is the width
of the copper circuit lines and the spacing between them. As the component density
and circuits per square inch increase, the width of the circuit lines and the spaces
between them must decrease. The current state of the art is 10 mil lines with 10 mil
spaces. This geometry is ultimately determined by the process technology that allows
the reliable fabrication of circuit boards within tolerances acceptable to the industry.
In normal production a 10 mil wide circuit line can be controlled to within plus or
minus 1 mil. If this line is spaced 10 mils from an adjacent line that may also vary
by only 1 mil, there is little chance of having broken lines or short circuiting between
lines. If, however, that line spacing geometry is reduced to 1 mil lines and spaces,
the previous tolerance is unacceptable and the process technology must be advanced
to achieve and maintain a tighter tolerance.
[0005] The most reliable and efficient method for connecting circuits on opposite sides
of a circuit board is the use of the plated through hole. Before the circuit pattern
is etched into the copper surfaces of the circuit board, the necessary interconnecting
points between the two sides are first located and a hole drilled through the board
at that position. In a complex circuit, there may be hundreds of holes each with its
own specification and tolerances, making the precision, quality and cleanliness of
each drill hole critical. Generally, after the circuit pattern has been defined, a
copper conductor is plated inside along the wall of each hole connecting the copper
circuits on one side to the other. To provide a good plated connection this hole must
be clean and free of any photoresist or other contaminant. The plated connection must
be almost perfect because each hole diameter is reduced by the thickness of the copper
plating. The remaining hole must be large enough for the leads of the circuit board's
components to be inserted into it but not too large to be filled with solder for a
final connection.
[0006] Two methods are commonly used for applying photoresist to the copper surface of a
circuit board. One of these is coating and the other is lamination. In coating, a
fluid containing the photopolymer dissolved in solvent is applied to the copper surface
in a thin uniform layer. The solvent is evaporated away and a uniform film of photoresist
is deposited onto the copper surface. In lamination a previously coated and dried
film of photoresist on a carrier web is bonded to the copper surface using heat and
pressure, after which the carrier web is stripped away.
[0007] Most of the circuit boards produced today use the dry film method primarily for these
two reasons. First there is no solvent fluid to cause safety, personnel, environmental
or disposal problems. Secondly, there is no liquid photoresist to get inside the drill
through holes to contaminate them and jeopardize the integrity of the plated through
connections. These two advantages of dry film over coating are substantial but they
are obtained at a price. One price is economic as dry film is about three times as
costly per square foot as a coated photoresist. The other price, far more costly,
is technological. Dry film has not been able to be produced reliably below a thickness
of one mil. In order to reduce the line spacing geometry so that circuit density can
be significantly increased, it is necessary to reduce the thickness of the photoresist
to around .1 and .2 mils. A reliable well adhered photoresist can only be applied
in that thickness range by using liquid coating technology. Handling the solvents
associated with liquid photoresists has already been solved, however, before any liquid
coating technology can be reliably used, the holes in the circuit board laminate must
be temporarily sealed.
[0008] There are three major problems to be overcome in the temporary sealing of the holes
in printed circuit board laminates before they are coated with photoresist. First,
the sealing material must prevent any significant amount of photoresist from entering
the hole. Secondly, the sealing material must be chemically inert, mechanically rugged
and adhered strongly enough to maintain its sealing function during the subsequent
processing steps. Finally and most importantly, the sealant material must be completely
removed from every hole without leaving any contamination or residue on the hole wall.
[0009] Various methods for temporarily sealing the holes have been proposed. Early work
described in U.S. Patent No. 2,965,952 related to filling the holes with an inert
material such as a protein substance. As pointed out in this patent, the process of
filling and later cleansing of the plated holes was very time consuming and required
a plating process in order to insure that the filling material would reside only in
the holes and not on the surfaces of the plate. U.S. Patent 2,965,952 attempts to
solve the filling and cleansing problem by employing decalcomanias as a resist to
the action of the etching solution. The decalomanias comprised a plurality of adhesive-backed
strips that extended over the holes in the printed circuit board. This procedure required
accurate registration of the protective strips and was also time consuming. U.S. Patent
No. 3,725,215 described a filler masking of small apertures in the printed circuit
boards. The process utilized a photo-hardenable material that was placed in the holes
with a backing sheet to prevent the material from falling through. After filling the
holes, the photo-hardenable material was exposed to radiant energy to solidify it.
Again, this process was time consuming and required care that the photo-hardenable
material did not extend beyond the perimeter holes themselves.
[0010] It is accordingly a general object of the present invention to provide an improved
method and apparatus for temporarily sealing holes in printed circuit board laminates.
[0011] It is a specific object of the invention to provide an apparatus which utilizes a
thermodeformable sheet material that can be deformed into the holes of the printed
circuit board under heat and pressure to form protective sealing plugs therein that
can be easily removed from the printed circuit board after conventional processing
of the board.
[0012] It is a further object of the invention to provide a cost effective apparatus and
process for temporarily sealing printed circuit board holes in which the sealing material
can be re-used a number of times.
[0013] It is a feature of the invention that the apparatus can be fabricated from commerical,
readily available components.
BRIEF DESCRIPTION OF THE INVENTION
[0014] The method of the present invention solves the three hole filling problems described
above by using a thermodeformable deformable sheet material that is deformed under
heat and pressure to form protectively sealing plugs in each of the holes of the printed
circuit board.
[0015] Deformation of the thermodeformable sheet material is performed in the apparatus
of the present invention. The apparatus comprises two major assemblies: a heater block
assembly and a vacuum table assembly. The heater block assembly employs a temperature
controlled, insulated heater block having a surrounding, movable vacuum collar. A
flexible, thermally conductive vacuum skin is mounted on the movable vacuum collar
and forms a vacuum chamber in cooperation with a movable vacuum table having a flat,
porous plate whose upper surface is parallel to the vacuum table surface. A sandwich
comprising a porous release sheet, the printed circuit board, the thermodeformable
sheet material and a thermally conductive cover sheet is positioned on the vacuum
table porous plate. Thereafter, the vacuum table and sandwich are moved together so
that the table forms a sealing relation with the flexible, thermally conductive vacuum
skin. A vacuum is then drawn in the now formed vacuum chamber forcing the flexible,
thermally conductive vacuum skin against the sandwich to compress the porous release
sheet between the printed circuit board and the flat, porous plate. At this point,
the flexible, thermally conductive vacuum skin, sandwich and vacuum table are moved
into a position in which the flexible, thermally conductive vacuum skin is forced
into thermally conductive contact with the heater block. Heat from the heater block
softens the thermodeformable sheet material so that it deforms into the holes of the
printed circuit board forming protectively sealing plugs therein. After cooling, the
vacuum table is moved in the reverse direction to permit removal of the sandwich.
The porous release sheet and cover sheet are then separated from the printed circuit
board and its protective, now deformed, thermodeformable sheet material. Conventional
processing of the PCB can be performed after which the deformed thermodeformable sheet
material is separated from the processed PCB for scrapping or re-use if desired.
[0016] The above described objects and features of the present invention and other objects
will best be understood from a detailed description of a preferred embodiment of the
invention, selected for purposes of illustration, and shown in the accompany drawings
in which:
Figures 1A through 1E illustrate in a generalized form the steps employed in the method
of the present invention to temporarily seal holes in a printed circuit board laminate;
Figure 2 illustrates diagramatically and in partial cross-section the apparatus of
the present invention comprising a heater block assembly and a vacuum table assembly,
a sandwich comprising a porous release sheet, a printed circuit board having holes
therein, a thermodeformable sheet material and a thermally conductive cover sheet
in position on the flat, porous plate of a movable vacuum table;
Figure 3 illustrates the same components of Figure 2 with the vacuum table moved into
sealing contact with a flexible, thermally conductive vacuum skin that together with
the vacuum table forms a vacuum chamber within which is positioned the PCB sandwich;
Figure 4 shows the same components of Figure 2 with the vacuum table moved into a
position in which the flexible, thermally conductive vacuum skin is forced into thermally
conductive contact with the heater block so that heat therefrom can soften the thermodeformable
sheet material so that it deforms into the holes of the PCB to form protectively sealing
plugs therein; and,
Figure 5 shows the vacuum table retracted from the heater block to permit removal
of the PCB sandwich.
DETAILED DESCRIPTION OF THE INVENTION
[0017] Turning now to the Figures, and specifically Figures 1A through 1E, there is shown
in generalized form the steps employed in practicing the method of the present invention.
A double-sided, copper clad laminate printed circuit board 10 having a plurality of
drill through holes 12 is placed on a vacuum table 14 having a flat, porous plate
16. A thermodeformable sheet material 18 is placed in contact with the upper surface
as viewed in of the printed circuit board 10. Heat is applied to the thermodeformable
sheet material and a vacuum is drawn through the flat, porous plate 16 to deform the
sheet material under heat and the vacuum force down into the drill through holes 12
to form protectively sealing plugs 20 therein as shown in Figure 1B.
[0018] If desired, a relatively stiff, adhesive bonded backing support 22 can be affixed
to the deformed sheet material 18 as shown in Figure 1C. The protectively sealed printed
circuit board laminate is turned upside down to expose the side of the printed circuit
board laminate that was in contact with the flat, porous plate 16 as shown in Figure
1D. This side of the printed circuit board is coated with a conventional photoresist
24. After exposure and development of photoresist 24, the deformed sheet material
20 and its support backing 22 are removed from the printed circuit board which now
contains developed photoresist 26.
[0019] Turning now to Figure 2, there is shown diagramatically and in partial section the
apparatus of the present invention used to practice the method thereof. The hole sealing
apparatus comprises two major assemblies: a heater block assembly indicated generally
by the reference numeral 28 and a vacuum table assembly indicated generally by the
reference numeral 30.
[0020] The heater block assembly 28 comprises a temperature controlled heater block 32 which
is insulated by a thermal insulator 34. The insulated heater block 32 is surrounded
by a movable vacuum collar 36 having a flexible, thermally conductive vacuum skin
38, such as, extra heavy duty aluminum foil, secured thereto and covering the heater
block 32. The material selected for the vacuum skin 38 preferably should be relatively
inexpensive and easily replaceable. Aluminum foil meets these criteria.
[0021] The vacuum collar 36 is mounted on collar air cylinders 40 which move the vacuum
collar and the covering flexible thermally conductive vacuum skin 38 to an extended
position as shown in Figure 2. The extended position of the vacuum collar is achieved
when a predetermined pressure of X psi is supplied to the collar air cylinders from
a regulated pressure source indicated generally by the reference numeral 40.
[0022] The vacuum table assembly 30 is located directly beneath the heater block assembly
28 and is movable with respect thereto. The vacuum table assembly comprises a vacuum
table 42 having a flat porous plate 44 mounted thereon so that the upper surface of
the porous plate is parallel to the surface of the vacuum table, an "O"-ring 46 surrounds
the porous plate and is positioned directly below the vacuum collar 36 of the heater
block assembly. Movement of the vacuum table 42 in the vertical direction, as shown
in Figure 2, is controlled by a main air cylinder 48 that is selectively coupled through
valves A and B to a low pressure source indicated generally by the reference numeral
50 and a high pressure source indicated generally by the reference numeral 52.
[0023] A vacuum source indicated generally by the reference numeral 54 and comprising a
vaccum pump 56 and vacuum valve 58 is connected to chamber 60 of the vacuum table.
The vacuum pump is capable of maintaining at least 25" Hg vacuum within chamber 60.
[0024] A sandwich indicated generally by the reference numeral 62 comprising a porous release
sheet 64, a printed circuit board laminate 66 having holes 68 therein, a thermodeformable
sheet material 70 and a thermally conductive cover sheet 72 is positioned on the porous
plate of the vacuum table.
[0025] Figure 2 illustrates the loading position of the vacuum table with valve A positioned
to exhaust the pressure from the main air cylinder 48 thereby lowering the vacuum
table away from the heater block assembly 28.
[0026] The sequence of operation once the sandwich 62 has been loaded is shown in Figures
3 through 5. Referring to Figure 3, valve B is positioned to connect the low-pressure
source (less than X psi) through valve A which is turned to send it to the main air
cylinder 48. The low pressure applied to the main air cylinder 48 raises the vacuum
table upwardly as viewed in Figure 3 until the "O"-ring 46 comes in contact with the
flexible, thermally conductive vacuum skin 38 covering vacuum collar 36. Upwardly
movement is continued slightly to compress the "O"-ring as shown in Figure 3 thereby
creating a vacuum seal between the vacuum skin 38 and the vacuum table. The vacuum
skin 38 and vacuum table 42 together form a vacuum chamber 74.
[0027] The low pressure air (less than X psi), is set so that it compresses the "O"-ring
against the vacuum skin 38, but cannot overcome the air pressure (X psi) applied to
the air collar cylinders 40.
[0028] When the vacuum valve 58 is turned from the vent position, as shown in Figure 2,
to the vacuum position, as shown in Figure 3, the vacuum pump 56 removes the air from
beneath the flexible skin 38 until a vacuum of 25" Hg is established as indicated
on a vacuum gauge 61. The vacuum causes the flexible vacuum skin 38 to conform around
the sandwich 62 forcing the layered components of the sandwich against the flat, porous
plate 44 of the vacuum table.
[0029] When sufficient vacuum has been established, valve B is turned so that the high pressure
air source 52 (greater than X psi) is connected to the main air cylinder 48 as shown
in Figure 4. The high pressure air allows the main air cylinder to overcome the pressure
in collar air cylinders 40 so that the collar is forced upwardly as viewed in Figure
4 allowing the heater block 32 to forcibly contact the flexible, thermally conductive
vacuum skin 38. Heat is transmitted from the heater block through the flexible, thermally
conductive vacuum skin and thermally conductive cover sheet 72 to the thermodeformable
material 70. The vaccum and air cylinder forces pushing the flexible vacuum skin and
thermally conductive cover sheet against the softened thermodeformable material force
the material down into the laminate's holes 68 and up against the release sheet 64
thereby forming a protectively sealing plug 76 in each of the PCB's holes.
[0030] When sufficient time has elapsed, valve B is turned to the low pressure air source
50 (less than X psi) to connect the low pressure source to the main cylinder. This
allows the pressure in the collar air cylinders 40 to force the main air cylinder
down, moving the flexible, thermally conductive vacuum skin 38 away from and out of
contact with the heater block 32 as shown in Figure 3. When the sandwich beneath the
vacuum skin has cooled sufficiently, the vacuum valve 58 is moved back to the vent
position as shown in Figure 5 and valve A is turned to the exhaust position, as shown
in Figure 5, to exhaust the air from the main cylinder. With the vacuum valve in the
vent position and valve A in the exhaust position, the vacuum table drops back down
to the loading position where the sealed sandwich can be removed. The process is then
repeated to seal another copper clad circuit board.
[0031] If the sandwich 62 has a non-uniform thickness as the result of non-planarity of
the printed circuit board laminate, variations in the thickness of the thermodeformable
sheet material, etc., contact between the heater block and the flexible, thermally
conductive vacuum skin (and therefor the sandwich) will not be uniform. The non-uniform
contact can produce relatively "hot" or "cold" spots with respect to the heat transferred
to the sandwich through the flexible, thermally conductive vacuum skin. The heat variations
can be eliminated by providing a liquid heat transfer medium, such as oil, in at least
a portion of the chamber 78 formed by collar 36, vacuum skin 38, air cylinders 40
and the heater block 32.
[0032] Having described in detail a preferred embodiment of my invention, it will be apparent
to those skilled in the art that various modifications can be made therein without
departing from the scope of the invention as described in the appending claims:
1. A thermo-vacuum apparatus comprising:
A. a heater block assembly comprising:
(1) a temperature controlled, insulated heater block means;
(2) a vacuum collar means surrounding said heater block means;
(3) a flexible, thermally conductive vacuum skin means secured to said vacuum collar
means;
(4) means connected to said vacuum collar means for moving said collar means and said
flexible, thermally conductive vacuum skin means between an extended position and
a retracted position with said extended position locating the flexible, thermally
conductive vacuum skin in spaced relation with respect to said heater block means;
B. a vacuum table assembly comprising:
(1) vacuum table means;
(2) a porous plate means mounted on said vacuum table means so that the upper side
of the porous plate means is parallel with the surface of the vacuum table means;
(3) vacuum sealing means located on said vacuum table means for establishing a vacuum
sealing relation with the flexible, thermally conductive vacuum skin means of said
heater block assembly;
(4) means connected to said vacuum table means for moving said vacuum table means
from a retracted position to an intermediate vacuum sealing position and then to a
contact position and vice versa, with said sealing relation being established between
the vacuum sealing means and the flexible, thermally conductive vacuum skin of the
heater block assembly at said intermediate vacuum sealing position and said flexible,
thermally conductive vacuum skin being forced into thermally conductive contact with
said heater block means at said contact position;
C. control means for selectively controlling the movement of said vacuum collar moving
means and said vacuum table moving means.
D. means for selectively applying and releasing a vacuum to said vacuum table means.
2. A thermo-vacuum apparatus comprising:
A. a heater block assembly comprising:
(1) a temperature controlled, insulated heater block means;
(2) a vacuum collar means surrounding said heater block means;
(3) a flexible, thermally conductive vacuum skin means secured to said vacuum collar
means;
(4) fluid actuated cylinder means connected to said vacuum collar means for moving
said collar means and said flexible, thermally conductive vacuum skin means between
an extended position and a retracted position with movement to said extended position
being in response to the application of fluid at a pressure X to said fluid actuated
cylinder means with said extended position locating the flexible, thermally conductive
vacuum skin in spaced relation with respect to said heater block means;
B. a vacuum table assembly comprising:
(1) vacuum table means;
(2) a flat, porous plate means mounted on said vacuum table means so that the upper
side of the porous plate means is parallel with the surface of the vacuum table means;
(3) vacuum sealing means located on said vacuum table means for establishing a vacuum
sealing relation with the flexible, thermally conductive vacuum skin means of said
heater block assembly;
(4) fluid actuated main cylinder means connected to said vacuum table means for moving
said vacuum table means from a retracted position to an intermediate vacuum sealing
position and then to a contact position and vice versa,
said fluid actuated main cylinder means moving said vacuum table means to said intermediate
sealing position in response to the application of a fluid at a pressure less than
X to said fluid actuated main cylinder means at which position said sealing relation
is established between the vacuum sealing means and the flexible, thermally conductive
vacuum skin of the heater block assembly and,
said fluid actuated main cylinder means moving said vacuum table means from said intermediate
sealing position to said contact position in response to the application of a fluid
at a pressure greater than X to said fluid actuated main cylinder means at which position
said flexible, thermally conductive vacuum skin is forced into thermally conductive
contact with said heater block means;
C. fluid pressure source means for applying fluid pressure at a pressure X to said
heater block assembly fluid actuated cylinder means;
D. fluid pressure source means for sequentially applying fluid pressure at a pressure
less than X and then at a pressure greater than X to said vacuum table assembly fluid
actuated main cylinder means said fluid pressure source means including means for
releasing the fluid pressure applied to said vacuum table assembly fluid actuated
main cylinder means; and,
E. means for selectively applying and releasing a vacuum to said vacuum table means.
3. The apparatus of claims 1 or 2 wherein said porous plate means is thermally non-conductive.
4. The apparatus of claim 3 wherein said thermally non-conductive porous plate means
comprises a porous ceramic plate.
5. The apparatus of claims 1 or 2 wherein said vacuum sealing means comprises a compressible
"O"-ring mounted on said vacuum table.
6. The apparatus of claims 1 or 2 wherein said vacuum sealing means comprises a compressible
"O"-ring mounted on said vacuum table and wherein said"O"-ring is compressed at said
intermediate sealing position to provide vacuum sealing between the vacuum table and
the flexible, thermally conductive vacuum skin of the heater block assembly.
7. The apparatus of claims 1 or 2 wherein the heater block means, vacuum collar means
and said flexible, thermally conductive vacuum skin means define a chamber and wherein
said chamber is at least partially filled with a liquid heat transfer medium.
8. A method for temporarily sealing holes in a printed circuit board laminate, said
method comprising the steps of:
A. forming a sandwich comprising:
(i) a first component having a melting point, said first component comprising a flexible,
thermally conductive sheet material;
(ii) a second component having a melting point that is lower than the melting point
of said first component, said second component comprising a sheet of thermodeformable
material;
(iii) a printed circuit board laminate having holes therein, said printed circuit
board being positioned with the second component sheet of thermodeformable material
in contact with one side of the printed circuit board laminate and covering at least
some of said holes; and,
(iv) a compressible porous release sheet positioned in contact with the other side
of the printed circuit board laminate;
B. placing said sandwich in a thermo-vacuum apparatus comprising:
(1) a heater block assembly comprising:
(a) a temperature controlled, insulated heater block means;
(b) a vacuum collar means surrounding said heater block means;
(c) a flexible, thermally conductive vacuum skin means secured to said vacuum collar
means;
(d) fluid actuated cylinder means connected to said vacuum collar means for moving
said collar means and said flexible, thermally conductive vacuum skin means between
an extended position and a retracted position with movement to said extended position
being in response to the application of fluid at a pressure X to said fluid actuated
cylinder means with said extended position locating the flexible, thermally conductive
vacuum skin in spaced relation with respect to said heater block means;
(2) a vacuum table assembly comprising:
(a) vacuum table means;
(b) a flat, porous plate means mounted on said vacuum table means so that the upper
side of the porous plate means is parallel with the surface of the vacuum table means;
(c) vacuum sealing means located on said vacuum table means for establishing a vacuum
sealing relation with the flexible, thermally conductive vacuum skin means of said
heater block assembly;
(d) fluid actuated main cylinder means connected to said vacuum table means for moving
said vacuum table means from a retracted position to an intermediate vacuum sealing
position and then to a contact position and vice versa,
said fluid actuated main cylinder means moving said vacuum table means to said intermediate
sealing position in response to the application of a fluid at a pressure less than
X to said fluid actuated main cylinder means at which position said sealing relation
is established between the vacuum sealing means and the flexible, thermally conductive
vacuum skin of the heater block assembly and,
said fluid actuated main cylinder means moving said vacuum table means from said intermediate
sealing position to said contact position in response to the application of a fluid
at a pressure greater than X to said fluid actuated main cylinder means at which position
said flexible, thermally conductive vacuum skin is forced into thermally conductive
contact with said heater block means;
3. fluid pressure source means for applying fluid pressure at a pressure X to said
heater block assembly fluid actuated cylinder means;
4. fluid pressure source means for sequentially applying fluid pressure at a pressure
less than X and then at a pressure greater than X to said vacuum table assembly fluid
actuated main cylinder means said fluid pressure source means including means for
releasing the fluid pressure applied to said vacuum table assembly fluid actuated
main cylinder means; and,
5. means for selectively applying and releasing a vacuum to said vacuum table means.
C. applying fluid pressure at a pressure X to said heater block assembly fluid actuated
cylinder means;
D. applying fluid pressure at a pressure less than X to the vacuum table assembly
fluid actuated main cylinder means so that the vacuum sealing means establishes said
vacuum sealing relation with the flexible, thermally conductive vacuum skin means
of the heater block assembly whereby a vacuum chamber is formed within which is located
said sandwich;
E. drawing a vacuum in said vacuumm chamber so that the flexible, thermally conductive
vacuum skin conforms around the sandwich whereby the layered components of the sandwich
are forced against the flat, porous plate means;
F. applying fluid pressure at a pressure greater than X to the vacuum table assembly
fluid actuated main cylinder means whereby the heater block means forcibly contacts
the flexible, thermally conductive vacuum skin;
G. transferring heat from said heater block means to the thermally deformable sheet
material until the sheet material has deformed into and filled the holes of the printed
circuit board laminate;
H. reducing the fluid pressure applied to the vacuum table assembly main cylinder
means to a pressure less than X so that the vacuum table means returns to said intermediate
position to permit cooling of the sandwich;
I. releasing the vacuum in said vacuum chamber and removing the fluid pressure applied
to said vacuum table assembly fluid actuated main cylinder means to allow the vacuum
table means to return to its retracted position; and,
J. removing the sandwich from the vacuum table.